1. Field of the Invention
The invention pertains to the field of electric motor control. More particularly, the invention pertains to control of two-phase motors.
2. Description of Related Art
In many examples of automated machinery, three-phase motors and two-phase motors are used to apply kinetic power to position loads, with machine builders mixing both types within a machine or across a number of machine models. In most instances, the 3-phase motors are permanent magnet servo motors used with feedback sensors to form a closed-loop device, while the two-phase motors are permanent magnet motors used in a stepping motor configuration which, are operated in an open-loop mode. Both can be used successfully to position loads within the machine.
Two-phase motors can be used as servo motors with a feedback device, and three-phase motors can be used in an open loop stepping mode, however, worldwide there are very few examples of these. Most servo motors and their drives are three-phase and most stepper motors and drives are two-phase.
Closed loop servo motors have the advantage of higher power output, enhanced precision, and assurance of operation when compared to open loop stepper motors, while open-loop stepper motors have the advantage of lower complexity and lower installed cost.
Modern electronic drives used to power two-phase motors use two “full-bridge” output circuits containing a total of eight power switching elements, while three-phase drives use three “half-bridges” which contain a total of six power switching elements. In both cases, modern motor drives typically contain a computing processor which directs the switching of the output elements based upon a command from a machine's high-level control electronics (and the motor's feedback device, if used).
With the current technology, manufacturers of automated machines have to purchase and stock two different types of motor drives for use in their machines if they wish to take advantage of the high performance of the servo motor for their demanding movement requirements while gaining the low cost/complexity benefits of the stepper motor for their less demanding movement requirements.
It would be an improvement to have one motor drive be capable of driving either two-phase or three-phase motors without adding to the cost or complexity of the motor drive circuitry. This would allow machine builders to integrate, stock and support one motor drive type instead of two, reducing their total costs and it would also allow the motor drive manufactures to increase the number of units sold of a specific type, increasing their economy of scale. It would further be an improvement if this could be done without increasing the base cost of the motor drive electronics.
There is a need for a single electronic motor drive configurable to run either two-phase or three-phase motor loads without any significant increase in the circuitry used to manufacture such a motor drive.
How a three-phase output stage (three half-bridges) operates a three-phase servomotor is well understood and taught by many references including U.S. Pat. Nos. 4,782,272, 4,208,621, and 4,814,677. Operating a two-phase motor with two half bridges is taught by many references including U.S. Pat. Nos. 4,490,664, and 6,016,044. Driving a three-phase stepper motor is taught by many references including U.S. Pat. Nos. 3,659,177, and 3,991,355.
FIG. 1 illustrates the prevailing prior art for driving a two-phase high-power stepping motor. Shown are two full-bridge stages (6, 9) (requiring eight switching elements total—S1A, S2A, S3A, S4A, S1B, S2B, S3B, S4B) driving two independent motor phase windings (A, B) of motor (2). This configuration may drive the motor with simple on and off sequencing of the switches (often referred to as full-step mode) or by modulating the switch's duty cycle so that currents can be arbitrarily controlled in the two motor phase windings (A, B) to position a shaft of the motor (2) between steps. When the phase currents iA and iB are controlled in a sinusoidal manner this is often referred to as micro-stepping mode.
Typically, to achieve a high shaft output power and precision, the current in each motor phase winding (A, B) is monitored by current sensors (7, 8) and controlled by a 2-phase PWM current controller (10) which utilizes a 2-phase Voltage Calculator (18) to produce the desired motor phase winding voltages VA and VB. VA and VB then control the full-bridge stages (6) and (9) producing switch control signals (3) via the full-bridge PWM modulators (24) and (25). In open loop (stepping) mode the current command (4) to the 2-phase PWM current controller (10) is most often constant during motor running and the angle command (5) is incremented to control the movement of the shaft of motor (2). Alternate methods to measure the currents iA and iB in the motor phase windings (A, B) may use sense resistors within the full-bridge stages (6, 9). Full voltage from the DC source (1) can be applied arbitrary to either or both motor phase windings (2) in either polarity, so the average peak-to-peak voltage across either motor phase winding can be as high as 2VDC.
FIG. 2 illustrates the prevailing prior art for driving three-phase motors. Three half-bridge output stages (13), (14) and (15) (requiring six switching elements total—S1R, S2R, S1T, S2T, S1S, S2S) are modulated by the 3-Phase PWM Current Controller (12) through switch control signals (11). To achieve a high shaft power from the motor (16) with minimum torque variations, the current (iR, iS) in the motor phase windings (R, T, S) are monitored by current sensors (7) and (8) and controlled based upon the shaft angle of the motor (16) which may be measured and fed to the Angle Command (5). (The current (iT) in the non-measured T phase being calculated as the sum of the measured currents by Kirchoff's law.) When the currents are controlled sinusoidally based upon the shaft angle, the control method is referred to by those skilled in the art as sine wave drive or vector control, depending on the internal operation of the 3-Phase PWM Current Controller (12). The current command (4) is then used to control the shaft torque output.
A Voltage Calculator is herein defined as the portion of the motor current control system that is responsible for generating the output voltage demand across the motor phase windings. A Voltage Calculator generally takes inputs from the motor phase current target inputs (4) and (5) and the phase current feedback signals (27) and (28), bus current feedback, or switching element current feedback but may operate with no current feedback whatsoever. The phase current target inputs to the Voltage Calculator most often employed are the vector amplitude of the current in the motor phases (Current Command (4)) and the angle of the current (Angle Command (5)), where the angle is used to set the target distribution between the phases. It will be understood that the target inputs may take on other forms, including the independent phase current values, etc.
Methods used within the current controller for determining the phase voltage outputs are wide and varied, i.e., there are many types of Voltage Calculators (18) or (20) known to the art. The exact internal operation of the Voltage Calculator (18) or (20) does not form part of the present invention. The following is a discussion of relevant background on voltage calculation which it is believed would enable one of ordinary skill in the art to design a voltage calculator (18) or (20) without undue experimentation.
Some Voltage Calculators use sine functions of the motor shaft angle as references to be compared against the phase currents, other examples use simple switching (square wave) functions or even arbitrary functions based upon the shaft angle as current references. Motor phase voltages are calculated based upon the difference between the reference currents and the measured or estimated phase currents.
Adding to the variation, these Voltage Calculators usually use current feedback sensors (7) and (8) as shown, but may not (there are also methods to estimate phase currents from the VDC bus current or combinations of the individual switching element currents) or current may not even be the controlled variable in simple systems where only voltages are used.
The voltages may be calculated directly from the difference in the individual phase current (direct method) or the phase voltages may be calculated as a group (or vector) after the phase currents are transformed into a different coordinate system (indirect method). All of these methods can be mixed and matched to form a Voltage Calculator.
FIG. 7 shows a block diagram of a 2-phase Voltage Calculator as known to the prior art. Referring to FIG. 7, the angle command input (5) is processed by a sin function (60), which outputs the sine of the angle command (5). The output of sin function (60) is multiplied in multiplier (62) by the current command (4), and then the current feed back iAsense (27) is subtracted from it in adder (64). The output of the adder (64) is input to integrator (66) and also scaled by a constant Ki in multiplier (68). The integrator (66) output is scaled by a constant K11 in multiplier (70), and then the scaled outputs from multipliers (68) and (70) are summed in adder (72), to form motor phase winding voltage VA.
Similarly, the angle command input (5) is processed by a cos function (61), which outputs the cosine of the angle command (5). The output of cos function (61) is multiplied in multiplier (63) by the current command (4), and then the current feed back iBsense (28) is subtracted from it in adder (65). The output of the adder (65) is input to integrator (67) and also scaled by a constant Ki in multiplier (69). The integrator (67) output is scaled by a constant K11 in multiplier (71), and then the scaled outputs from multipliers (69) and (71) are summed in adder (73), to form motor phase winding voltage VB.
It will be understood that the diagram of FIG. 7 is exemplary only, and the structure of the Voltage Calculators employed, be they for three-phase motor control or two-phase motor control, are prior art that is outside the scope of the present invention.